Communication device and method providing beamforming for two or more transmission channels

ABSTRACT

A control unit of a communication device provides multicast precoding information from at least first beamforming information descriptive for a first transmission channel and second beamforming information descriptive for a second transmission channel. A precoder unit beamforms at least one signal using the multicast precoding information to obtain at least two precoded signals. A transmitter circuit which is electrically coupled to the precoder unit multicasts transmission signals through the at least first and a second transmission channels, wherein the transmission signals are derived from the precoded signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on PCT/EP2014/000584 filed Mar. 6,2014, and claims priority to European Patent Application 13001648.8,filed in the European Patent Office on Mar. 28, 2013, the entirecontents of each of which being incorporated herein by reference.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to a communication device to beamform asignal using a precoding matrix. The disclosure further relates to acommunication device receiving information on a precoding matrix, and acommunication system and a method of beamforming signals for thetransmission through transmission channels.

2. Description of Related Art

Beamforming improves MIMO (multiple input multiple output) and MISO(multiple input single output) communication significantly. Typically,beamforming is used to modify the beam or spot for obtaining maximumSNRs (signal-to-noise-ratio) or maximum data throughput rates in unicastor broadcast communication links. In wired as well as wirelesscommunication systems beamforming may also be used to minimize EMI(electromagnetic interference) at desired locations.

It is an object of the embodiments to provide communication devices foran improved communication system using beamforming as well as acommunication system and a method providing enhanced beamformingcapabilities.

SUMMARY

A control unit of a communication device provides multicast precodinginformation from at least first beamforming information descriptive fora first transmission channel and second beamforming informationdescriptive for a second transmission channel. A precoder unit beamformsat least one signal using the multicast precoding information to obtainat least two precoded signals. A transmitter circuit is electricallycoupled to the precoder unit and multicasts transmission signals throughthe at least first and second transmission channels, wherein thetransmission signals are derived from the precoded signals.

Another communication device includes a receiver circuit for receivingat least two multicast transmission signals. A decoder unit decodes theat least two transmission signals to obtain at least one decoded signal.The communication device is configured to transmit expanded beamforminginformation in response to a signal received through the receivercircuit.

A communication system includes at least three communication devices. Acontrol unit of at least one of the communication devices providesmulticast precoding information from at least first beamforminginformation descriptive for a first transmission channel and secondbeamforming information descriptive for a second transmission channel. Aprecoder unit beamforms at least one signal using the multicastprecoding information to obtain at least two precoded signals. Atransmitter circuit is electrically coupled to the precoder unit andmulticasts transmission signals through the at least first and secondtransmission channels, wherein the transmission signals are derived fromthe precoded signals.

A method of operating a communication device includes providingmulticast precoding information from at least first beamforminginformation descriptive for a first transmission channel and secondbeamforming information descriptive for a second transmission channel.At least one signal is beamformed using the multicast precodinginformation, wherein at least two precoded signals are obtained. Througha transmitter circuit which is electrically coupled to the precoder unittransmission signals derived from the precoded signals are multicastthrough the at least first and second transmission channels.

A communication device includes means for obtaining multicast precodinginformation from at least first beamforming information descriptive fora first transmission channel and second beamforming informationdescriptive for a second transmission channel, means for beamforming atleast one signal using the multicast precoding information to obtain atleast two precoded signals, and means for multicasting transmissionsignals derived from the precoded signals through the at least first andsecond transmission channels.

The foregoing paragraphs have been provided by way of generalintroduction and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings. The elements ofthe drawings are not necessarily to scale relative to each other. In thefollowing drawings, like reference numerals designate identical orcorresponding parts throughout the several views. Features of theillustrated embodiments can be combined with each other to form yetfurther embodiments.

FIG. 1A is a schematic block diagram of a communication system providingmulticast transmission according to an embodiment.

FIG. 1B is a schematic block diagram of a communication system accordingto an embodiment referring to wireless transmission.

FIG. 1C is a schematic block diagram of a PLC (power line communication)system in accordance with a further embodiment.

FIG. 1D is a schematic diagram for illustrating channel parameters of awireless communication system according to an embodiment.

FIG. 2A is a schematic block diagram of a communication device inaccordance with an embodiment providing multicasting of aEigenbeamformed signal.

FIG. 2B is a schematic block diagram of a communication device inaccordance with an embodiment providing multicasting of spot beamformedsignals.

FIG. 3A is a schematic diagram showing a beam map of a firstcommunication link of a communication system for illustrating effects ofthe embodiment.

FIG. 3B is a schematic diagram showing a beam map of a secondcommunication link of a communication system for illustrating effects ofthe embodiment.

FIG. 3C is a schematic diagram showing a beam map of a thirdcommunication link of a communication system for illustrating effects ofthe embodiment.

FIG. 4A is a schematic block diagram of a communication device inaccordance with an embodiment relying on expanded beamforminginformation.

FIG. 4B shows a schematic diagram illustrating an SNR difference betweenthe first and the second communication links of FIGS. 3A and 3B forillustrating a method of obtaining multicast precoding matrices usingthe communication device of FIG. 4A.

FIG. 4C shows a schematic diagram illustrating an SNR difference betweenthe first and the third communication links of FIGS. 3A and 3C forillustrating a method of obtaining multicast precoding matrices usingthe communication device of FIG. 4A.

FIG. 4D shows a schematic diagram illustrating an SNR difference betweenthe second and the third communication links of FIGS. 3B and 3C forillustrating a method of obtaining multicast precoding matrices usingthe communication device of FIG. 4A.

FIG. 4E is a schematic 3D plot of the beam maps of the first and thirdcommunication links of FIGS. 3A and 3C.

FIG. 5 is a schematic block diagram of a communication device inaccordance with an embodiment providing multicast transmission andbeamforming for PLC.

FIG. 6 is a simplified flowchart of a method of operating acommunication device.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1A illustrates a wireless or wired communication system 500. Forexample, the communication system 500 may be an xDSL (generic digitalsubscriber line) or a DVB-C2 (digital video broadcasting-cable)communication system, an ad-hoc network, for example a WLAN (wirelesslocal area network) or a network of sensor and/or actuator devices. Inaccordance with an embodiment, the communication system 500 is a systemusing power distribution wires for data communications. For example, thecommunication system 500 employs power line communication (PLC), mainscommunications, power line telecommunications (PLT), broadband powerline (BPL) or power band or power line networking (PLN) and is based ona modulated carrier superimposed to the 50 or 60 Hz alternating currentof the power lines. The communication system 500 may apply an OFDM(orthogonal frequency division multiplexing) modulation scheme.

The communication system 500 is a MIMO or MISO system with at least onetransmitting communication device 100 and at least two receivingcommunication devices 910, 920. A first communication link isestablished between the transmitting communication device 100 and afirst receiving communication device 910 and a second communication linkis established between the transmitting communication device 100 and asecond receiving communication device 920. Further communication linksmay be established between the transmitting communication device 100 andfurther receiving communication devices. Signals exchanged between thetransmitting communication device 100 and the receiving communicationdevice 910 pass a first transmission channel 510 and signals exchangedbetween the transmitting communication device 100 and the secondreceiving communication device 920 pass through a second transmissionchannel 520. The first and second transmission channels 510, 520 may bewired or wireless channels. The transmitting communication device 100may be an exclusively transmitting device with transmit ports only or atransceiver device including both transmitter and receiver units fortransmitting and receiving signals through the same physicaltransmission channel. The receiving communication devices 910, 920 maybe exclusively receiving devices or transceiver devices including bothreceiver and transmitter circuits for receiving and transmitting signalsthrough the same physical channel.

The transmitting and receiving communication devices 100, 910, 920 maybe stand-alone devices or may be integrated in electronic devices forconsumer applications, for example in storage units, television sets,audio receivers, home servers storing video or audio content, routersconnected to the internet, computers, video recorders, sensor devicesand actuator devices.

The hardware configuration of the communication devices 100, 910, 920may be almost identical, whereby the data transmission quantity may beasymmetric in the application. For example, a communication deviceintegrated in a server may output large data contents like video streamswhereas a communication device included in a television apparatus mayoutput only comparatively short requests and handshake messages.

The transmitting communication device 100 includes a control unit 110that provides multicast precoding information descriptive for amulticast precoding matrix from first beamforming informationdescriptive for the first transmission channel 510 and secondbeamforming information descriptive for the second transmission channel520. In case more communication links are established, the control unit110 may obtain the multicast precoding information by evaluating furtherbeamforming information descriptive for the further transmissionchannels. The control unit 110 may be implemented in hardware as acontrol circuit including or consisting of one or more ICs (integratedcircuit), FPGAs (field programmable array), GALs (generic-array oflogic), ASICs (application specific integrated circuit) and theirequivalents. According to another embodiment the control unit 110 may berealized completely in software running, e.g., in a DSP (digital signalprocessor) or in an embedded system.

A precoder unit 150 uses a multicast precoding matrix obtained from themulticast precoding information to beamform one, two or more signals andoutputs at least two precoded signals obtained by precoding the one, twoor more signals. The signals are any transmissions sent to therespective transmission channel. The signals may include communicationdata, system synchronization information or any test signals e.g. usedfor channel or noise estimation. The precoder unit 150 may beimplemented in hardware as a precoder circuit electrically coupled tothe control unit 110 and including or consisting of one or more ICs,FPGAs, GALs, ASICs and their equivalents. The control and precoder units110, 150 may be integrated in the same IC housing or may share the samecircuitry. According to another embodiment the precoder unit 150 isrealized completely in software running, e.g., in a DSP (digital signalprocessor) or in an embedded system.

A transmitter circuit 190 receives the precoded signals and multicaststransmission signals through the first and second transmission channels510, 520, thereby establishing at least two multicast communicationlinks between the transmitting communication device 100 and thereceiving communication devices 910, 920 wherein the transmittingcommunication device 190 sends the same transmission signals to bothreceiving communication devices 910, 920. Accordingly, both receivingcommunication devices 910, 920 receive, process and evaluate the sametransmission signals. The transmitter circuit 190 may be implemented inhardware including or consisting of one or more ICs, FPGAs, GALs, ASICsand their equivalents. According to another embodiment the transmittercircuit 190 may include software running, e.g., in a DSP or an embeddedsystem. The control unit 110, the precoder unit 150 and the transmittercircuit 190 may share the same computational resources.

The beamforming information evaluated by the control unit 110 relatesone or more candidate matrices to one or more channel parametersdescriptive for the first and second transmission channels 510, 520.

According to an embodiment the channel parameters describe transmissiondirection and and transmission distance of the respective transmissionchannels 510, 520. According to another embodiment, the channelparameters include or consist of a channel quality criterion thatdescribes a transmission characteristic of the first and secondtransmission channels 510, 520 in case the respective candidate matrixis used for precoding a signal. The candidate matrices may be vectors orcombinations of beamforming parameters like beamforming angles.

The control unit 110 may in one embodiment provide the multicastprecoding matrix by using an^ algorithm considering whether or not oneof the transmission channels 510, 520 exhibits a better transmissionquality for all candidate matrices than the others.

When for at least a first one of the candidate matrices the channelquality criterion indicates in some respect a better transmissionquality through the first transmission channel 510 and for at least asecond one of the candidate matrices the channel quality criterionindicates in the same respect a better transmission quality through thesecond transmission channel 520, determining the multicast precodingmatrix includes in one embodiment identifying those candidate matricesthat provide approximately equal channel quality criteria for at leastthe first and second transmission channels 510, 520. According to anembodiment, determining the multicast precoding matrix may includeselecting from the candidate matrices providing approximately equalchannel quality criteria that candidate matrix providing the bestchannel quality.

The channel quality criterion may be the SNR, a number indicating arange in which the SNR falls, the maximum data throughput rate, a BER(bit error rate) or a number indicating a range of the BER. With thechannel quality criterion being the SNR, two candidate matrices areassumed to provide equal channel criteria when the SNRs deviate fromeach other by not more than a preset value, for example 3 dB. Accordingto an embodiment the preset value may be 0.5 dB. In case the channelquality criterion is an index identifying a preset range of the SNR, twocandidate matrices are assumed to provide equal channel criteria whenthey have the same range index. Equivalent considerations apply to BERand data throughput rate.

As a result, in cases when for at least a first one of the candidatematrices the channel quality criterion indicates a better transmissionquality through the first transmission channel 510 and for at least asecond one of the candidate matrices the channel quality criterionindicates a better transmission quality through the second transmissionchannel 520, in other words for more or less balanced transmissionchannels, the transmitting communication device 100 uses a precodingmatrix that ensures that in case of multicast transmission alltransmission channels 510, 520 exhibit the best common SNR. Thereceiving communication devices 910, 920 receive the multicasttransmission signal at the same quality level without wasting channelresources. Equalized SNR by beamforming results in that allcommunication links have identical SNR margins and therefore identicalQoS (quality of service) conditions. The embodiments allow multicasttransmission to benefit from beamforming. If the communication system500 uses QAM (quadrature amplitude modulation), identical constellationsmay be used in the multicast transmission for corresponding carriers.

Usually, in a communication system some of the communication links havebetter SNR allowing high constellations and other communication linkshave a low SNR allowing only low constellations. If the SNRs in theconcerned communication links differ from each other, the transmittingcommunication device 100 has to adjust the constellations in a way thatthe communication link exhibiting the lowest SNR is supported. Thereforesystem resources are wasted since the other links do not exploit thebetter transmission quality of their transmission channels. In case twoor more transmission channels 510, 520 have approximately equal SNRs theavailability of each transmission channel 510, 520 can be exploited to ahigher degree at least in cases with more or less balanced communicationlinks.

Otherwise, for strongly unbalanced systems with one of the transmissionchannels 510, 520 inferior to the others for all candidate matrices, thecandidate matrix with the best channel quality criterion for the mostinferior transmission channel may be selected as the multicast precodingmatrix.

Each of the receiving communication devices 910, 920 includes a receivercircuit 890 for receiving at least one multicast transmission signalsand a decoder unit 850 that applies a decoding matrix adapted to themulticast precoding matrix and the concerned transmission channel. Thedecoding matrices for decoding the multicast transmission signals in thefirst and second receiving communication devices 910, 920 may be equalor may differ from each other. For providing the adapted decodingmatrices, the transmitting communication device 100 may transmitinformation descriptive for the precoding matrix to the concernedreceiving communication devices 910, 920. According to anotherembodiment, the transmitting communication device 100 may applybeamforming on one or more types of non-payload data, for example burstpreambles, frame control signals, and delimiters. The concernedreceiving communication devices 910, 920 may perform an additionalchannel estimation phase for estimating the new equivalent channel,wherein the new equivalent channel takes account of both the multicastbeamforming and the actual transmission channel.

According to another embodiment the decoder unit 850 decodes the atleast two transmission signals to obtain at least one decoded signal andthe communication device 100 is configured to transmit expandedbeamforming information, for example in response to a test signalreceived through the receiver circuit 810.

FIG. 1B shows the communication system 500 of FIG. 1A for wirelessapplications. The communication system 500 may be an ad-hoc network, forexample a WLAN (wireless local area network) or a wireless network ofsensor and actuator devices. The communication links between the firstcommunication device 100 and the second communication devices 910, 920may be bidirectional.

FIG. 1C refers to an embodiment of the communication system 500 of FIG.1A with the transmission channels 510, 520 embodied by in-house electricpower wiring 350 including three or more electrical conductors used fortransmission of AC (alternating current) electric power and installed aspermanent wiring within buildings or buried in the ground. For example,the communication system 500 may include a transmitting communicationdevice 100 that may be integrated in or electrically coupled with, forexample, a home server containing and administering video, audio or datacontent in a first room 102. In a second room 912, a first receivingcommunication device 910 may be integrated in or electrically coupled toa router connected to the Internet. In a third room 922 a secondreceiving communication device 920 may be integrated in or electricallycoupled to a home computer and in a fourth room 932 a third receivingcommunication device 930 may be integrated in or electrically coupled toa television apparatus.

Line cords 511 plugged into power outlets 510 connect the communicationdevices 100, 910, 920, 930 with the in-house electric power wiring 350.Via the communication devices 100, 920, 930 the home server in the firstroom 102 may multicast a video stream to the home computer in the thirdroom 922 and the television apparatus in the fourth room 932, whereinthe multicast signal may include two or more contemporaneouslytransmitted transmission signals. The television apparatus and the homecomputer may receive, evaluate and process the transmission signal inthe same way.

For example, the transmitting communication device 100 may supply twodifferential transmission signals using the life or phase wire (L, P),the neutral wire (N), and protective earth (PE) wherein the differentialtransmission signals are modulated on a carrier superposing the ACfrequency of the mains voltage. The receiving communication devices 910,920, 930 may receive two or three differential receive signals betweenlife wire and neutral wire, between neutral wire and protective earth,and between life wire and protective earth. According to otherembodiments, the second communication devices 910, 920, 930 may receiveone or two differential receive signals together with a common modesignal resulting from a leakage current from the wiring as a furtherreceive signal.

FIG. 1D illustrates channel parameters for a wireless communicationsystem using beamforming. A transmission channel between thetransmitting communication device 100 and a receiving communicationdevice 910 may be described by the transmission angles α, β an thetransmission distance td, by way of example.

FIG. 2A refers to an embodiment providing Eigenbeamforming of twosignals, wherein each signal may be assigned to another content. Theprecoder unit 150 eigenbeamforms the signals using a multicast precodingmatrix. The precoder unit 150 outputs two or more precoded signals,wherein at least one of the precoded signals is a multicast signal. Thetransmitter circuit 190 receives the precoded signals and transmitstransmission signals obtained from the precoded signals. The controlunit 110 obtains multicast precoding information descriptive for amulticast precoding matrix from first and second beamforming informationdescriptive for the first and second transmission channels through whichthe multicast transmission signals are transmitted.

In case of Eigenbeamforming, the precoder matrix is a unitary matrix V.In case of two transmit ports and two signals, the unitary matrix V is acomplex 2×2 matrix whose entries can be derived from two beamformingangles θ and ψ as given by equation (1):

$\begin{matrix}{\underset{\_}{V} = {\begin{bmatrix}{\overset{arrow}{v}}_{1} & {\overset{arrow}{v}}_{2}\end{bmatrix} = {\begin{bmatrix}v_{11} & v_{12} \\v_{21} & v_{22}\end{bmatrix} = \begin{bmatrix}{\cos\;\psi} & {\sin\;\psi} \\{{- {\mathbb{e}}^{j\;\theta}}\sin\;\psi} & {{\mathbb{e}}^{j\;\theta}\cos\;\psi}\end{bmatrix}}}} & (1)\end{matrix}$

All possible precoding matrices including the candidate matrices can berepresented by combinations of the beamforming angles θ, ψ within theranges 0≦ψ≦π/2 and −π≦θ≦π.

In each receiving communication device 910, 920 a decoder unit decodesthe receive signals using a decoding matrix. Provided that a MIMOequalizer in the receiving communication devices is based on ZF(zero-forcing) detection, the detection matrix W is given by the pseudoinverse matrix H^(P) of the channel matrix H and the hermitian transposeV^(H) of the precoding matrix V as given by equation (2).W=V ^(H) H ^(P) =V ^(H)(H ^(H) H)⁻¹ H ^(H)  (2)

The SNR of the decoded receive signals is given by equations (3a) and(3b):

$\begin{matrix}{{SNR}_{1} = {\rho\frac{1}{{w_{1}}^{2}}}} & ( {3a} ) \\{{SNR}_{2} = {\rho\frac{1}{{w_{2}}^{2}}}} & ( {3b} )\end{matrix}$

In equations (3a), (3b) ρ gives a ratio of transmit power to noise powerand the terms ∥w₁∥ give the norm of the i-th row of the decoding matrixW. From this follows that the SNR depends on the beamforming angles θand ψ and the channel matrix H. The relationship between combinations ofthe beamforming angles θ and ψ and the SNR can be represented by beammaps. A complete beam map of a transmission channel assigns, to eachcombination or pairs of beamforming angles θ and ψ, the correspondingSNR.

According to an embodiment, only the dominant precoded signal providingthe highest SNR is used for multicast transmission and the second andfurther precoded signals are neglected in the process of determining themulticast precoding matrix. The second and further precoded signals mayalso be neglected for transmission. According to other embodiments, thesecond precoded signal may be used for transmitting a further signal,which may be a unicast signal.

FIG. 2B refers to an embodiment of the communication device 100providing spot beamforming. The precoder unit 150 beamforms one singlesignal to at least two beamformed signals. The transmitter circuit 190receives the at least two precoded signals and multicasts acorresponding number of transmission signals derived from the precodedsignals. In the case of a precoder unit 150 providing two spotbeamformed signals, the precoding matrix V as given in equation (4) isrepresented by a beamforming vector corresponding to the first columnvector of the matrix V of equation (1):

$\begin{matrix}{\underset{\_}{V} = {{\overset{arrow}{v}}_{1} = {\begin{bmatrix}v_{11} \\v_{21}\end{bmatrix} = \begin{bmatrix}{\cos\;\psi} \\{{- {\mathbb{e}}^{j\;\theta}}\sin\;\psi}\end{bmatrix}}}} & (4)\end{matrix}$

Here and in the following the description refers to two transmissionsignals derived from two precoded data signals for illustrativepurposes. Other embodiments may provide three or more precoded datasignals and a corresponding number of transmission signals.

The following Figures refer to embodiments providing expandedbeamforming information to the control units 110 of the precedingFigures, wherein expanded beamforming information contains moreinformation about a relationship between the beamforming angles θ, ψ anda channel quality criterion than a minimum beamforming information. Theminimum beamforming information describes not more than a specificprecoding matrix and one or more channel parameters linked to the use ofthe specific precoding matrix. According to an embodiment the channelparameter may be a channel quality criterion, e.g. the highest SNRpossible by beamforming, and the specific precoding matrix is that onerequired at the transmitter side for providing the channel qualitycriterion, e.g. that one required for achieving the highest SNR possibleby beamforming. For example, the specific precoding matrix may beobtained by singular value decomposition using channel impulse responseestimates. Other embodiments may rely on BER or data throughput as thechannel quality criterion or may link a specific precoding matrix withsuppressing noise or reducing EMI at certain locations. The followingembodiments refer to the SNR as an example for the channel qualitycriterion.

FIGS. 3A to 3C show three beam maps 301, 302, 303 for threecommunication links L1, L2, L3 between a transmitting communicationdevice 100 and three receiving communication devices 910, 920, 930 asillustrated in FIG. 1A. Each beam map shows the SNRs for everycombination of the beamforming angles θ, ψ in one of the communicationlinks L1, L2, L3 for an arbitrary selected frequency carrier. In eachbeam map the horizontal and vertical axes represent the beamformingangles θ, ψ. The contour lines connect combinations of the beamformingangles θ, ψ having the same SNR and are labeled with the pertinent SNRvalue in dB. Each beam map spans a 3D surface in the space defined bythe three axes defined by the first beamforming angle θ, the secondbeamforming angle ψ and the SNR.

The beamforming angles θ, ψ may be used as the entries of candidatematrices. Summarized, the complete beam maps 301, 302, 303 give the SNRfor any combination of the beamforming angles θ, ψ for all communicationlinks L1, L2, L3 in case of a unicast transmission. The combination ofbeamforming angles θ, ψ providing the highest SNR possible for therespective transmission channel by beamforming is marked with thediamond symbol. The diamond symbol indicates the combination ofbeamforming angles θ, ψ selected for beamforming with the best SNR or atmaximum data throughput rate for unicast transmission.

The beam maps are derived for each communication link L1, L2, L3 ortransmission channel and each frequency carrier separately, and the beammaps of different communication links or other frequency carrierstypically differ from each other. The beam maps for each communicationlink may be calculated using the beamformed signal optimized for any ofthe other communication links.

The beam maps change when the properties of the transmission channelchange, for example if in a mains grid a switch for an electric load istoggled. Accordingly a communication device may monitor the transmissionchannel to update the beam maps when required. Alternatively or inaddition the communication devices may update their beam mapsperiodically at predetermined intervals.

In case of an OFDM system using a plurality of spaced subcarriers, abeam map may be determined for each subcarrier frequency individually.Alternatively, the same beam map may be assigned to a group ofneighboring subcarriers. In addition or alternatively beam maps of somesubcarriers may be interpolated from the beam maps of adjacentsubcarriers.

Minimum beamforming information identifies not more than one point ofthe 3D surface of the beam map, e.g. the beamforming angles θ, ψassigned to the maximum SNR and the maximum SNR value. Expandedbeamforming information identifies at least two points of the 3Dsurface. According to an embodiment, the expanded beamforminginformation identifies at least location and value of the maximum SNRand location and SNR value of one or more further points of the 3Dsurface. The further points may be further local maxima and minima SNRs.According to other embodiments, the expanded beamforming information maycontain the SNRs for a predefined number of predefined pairs ofbeamforming angles θ, ψ, for example the SNRs assigned to intersectionpoints of an orthogonal grid, which may have equally spaced meshes. Theexpanded beamforming may include location and SNRs values for 2 to 256points on the 3D surface of the beam map, by way of example, wherein thesize of the feedback information per combination of the beamformingangles θ and ψ may be between 1 and 8 bits.

According to an embodiment the transmitting communication device 100 mayobtain the expanded beamforming information by transmitting signalstrying various combinations of the beamforming angles θ, ψ to each ofthe receiving communication devices 910, 920, 930 selected forparticipating in the multicast transmission. The receiving communicationdevices 910, 920, 930 may analyze a specific portion of signals thatcontain payload data, for example a preamble portion containing trainingsymbols, or pilot portions interspersed in the signal on specificfrequency carriers. Other embodiments may provide specific test signalsto this purpose. In response to the signals, each of the concernedreceiving communication devices 910, 920, 930 feedbacks informationdescriptive for the optimum combination of beamforming angles θ, ψ andan updated tone map including information descriptive for the SNRachieved with the optimum combination of beamforming angles θ, ψ. Thetransmitting communication device 100 may use the received informationfor gathering the expanded beamforming information or for looking forbetter choices for the combination of beamforming angles θ, ψ in asystematic manner. Since the approach gets by with feedback informationdefined in existing standards, the approach can be implemented on top ofany standard that already supports beamforming like Homeplug AV2 or G.hn9963.

Other embodiments may provide, as the expanded beamforming information,an index descriptive for one of several predefined candidate functionsand coefficients for locally modifying the indicated candidate function.The candidate functions may differ as regards topologicalcharacteristics of the 3D surface, e.g. number and relative orientationof local SNR maxima and SNR minima. By applying the coefficients, whichmay include local compression/stretching factors along the axes, to thecandidate function the resulting approximation function may approximatethe actual beam map.

FIGS. 4A to 4D refer to an embodiment based on receiving expandedbeamforming information. Expanded beamforming information contains moreinformation of the beam map of a communication link than minimuminformation, e.g. about the maximum SNR and the beamforming angles θ, ψassigned to the maximum SNR. For example, the expanded beamforminginformation contains in addition to the minimum information furtherinformation about position and value of selected points in the 3Dsurface of the beam map, for example information about location andvalues of local SNR minima and SNR maxima, the SNRs of regularly spacedpoints, or of points selected according to their information content.The expanded beamforming information may describe at least 8 points(3-bit feedback information size) of the 3D surface of the beam map, forexample up to 1024 points, wherein a size of feedback information forthe angles θ and ψ may be 10 bit. According to another embodiment theexpanded beamforming information may contain information descriptive fora mathematical expression approximating the actual beam map.

The control unit 110 of FIG. 4A receives first expanded beamforminginformation BMe1 related to a first communication link L1 and secondexpanded beamforming information BMe2 related to a second communicationlink L2. A comparator unit 111 may compare the SNRs of correspondingcombinations of the beamforming angles θ, ψ contained in or derived byway of interpolation or extrapolation from in the expanded beamforminginformation BMe1, BMe2. For example, the comparator unit 111 maysubtract the SNRs of the second expanded beamforming information BMe2from the corresponding SNRs of the first expanded beamforminginformation BMe1 and may check, for each available combination ofbeamforming angles θ, ψ, whether the resulting difference is below apredetermined threshold or, otherwise, the sign of the difference.

If for all combinations of the beamforming angles θ, ψ, for which theexpanded beamforming information contains information about the SNR, theSNRs of the first expanded beamforming information BMe1 exceed the SNRsof the second expanded beamforming information BMe2, then the secondcommunication link is always inferior to the first communication link.In this case a first selector unit 113 selects a combination ofbeamforming angles θ, ψ that is assigned to the highest SNR in thesecond expanded beamforming information BMe2 as the multicast precodingmatrix for multicast transmission on both communication links. If allSNRs of the second expanded beamforming information BMe2 exceed thecorresponding SNRs in the first expanded beamforming information BMe1,then for each combination of beamforming angles θ, ψ the firstcommunication link is inferior to the second communication link and thefirst selector unit 113 selects that combination of beamforming anglesθ, ψ which is assigned to the highest SNR in the first expandedbeamforming information BMe1 as the multicast precoding matrix.

If for at least one combination of beamforming angles θ, ψ thedifference of the corresponding SNRs is less than a predeterminedthreshold, for example less than 3 dB or less than 0.5 dB, then a secondselector unit 115 selects from all combinations of beamforming angles θ,ψ for which the SNRs of the two communication links deviate from eachother by not more than the predetermined threshold one with the highestSNR.

Each of the comparator unit 111, the first selector unit 113 and thesecond selector unit 115 may be implemented in hardware as comparatorcircuit, first selector circuit and second selector circuit, each ofthem including or consisting of one or more ICs, FPGAs, GALs, ASICs andtheir equivalents. According to another embodiment one, two or all ofthe comparator unit 111, the first selector unit 113 and the secondselector unit 115 may include software running, e.g., in a DSP or anembedded system.

FIGS. 4B to 4D refer to the communication links L1, L2, L3 shown inFIGS. 3A to 3C. The contour lines indicate combinations of beamformingangles θ, ψ for which the SNRs of the concerned communication links havethe same distance to each other. The numbers attached to the contourlines give the SNR difference in dB.

The diagram in FIG. 4B shows the result of subtracting the beam map ofthe second communication link L2 from the beam map of the firstcommunication link L1. The resulting SNRs are all negative indicatingthat the first communication link L1 is the inferior of both. Thebeamforming angles θ=−0.9 and ψ=0.55 indicating in FIG. 3A the maximumSNR in the first communication link L1 are selected for a multicasttransmission through the first and second communication links L1, L2.

The diagram of FIG. 4C shows the result of subtracting the complete beammap of the third communication link L3 of FIG. 3C from the complete beammap of the first communication link L1 of FIG. 3A. FIG. 4C reveals thatfor first combinations of beamforming angles θ, ψ the SNR differencesare negative indicating that the first communication link L1 is inferiorto the third communication link L3 and that for second combinations ofbeamforming angles θ, ψ the SNR differences are positive indicating thatthe third communication link L3 is inferior to the first communicationlink L1. Along a line 310 the SNR differences are equal zero. One of thecombinations of beamforming angles θ, ψ along line 310 may be selectedas the multicast precoding matrix to ensure equal QoS for a multicasttransmission through the first and third communication links L1, L3.

The diagram of FIG. 4D shows the result of subtracting the complete beammap of the third communication link L3 of FIG. 3C from the complete beammap of the second communication link L2 of FIG. 3B. All SNR values arepositive indicating that the third communication link L3 is inferior tothe second communication link L2. The beamforming angles θ=−0.35 andψ=0.65 indicating in FIG. 3C the maximum SNR in the third communicationlink L3 are selected for a multicast transmission through the second andthird communication links L2, L3.

FIG. 4E is a 3D diagram showing the beam maps for the first and thirdcommunication links L1, L3. Line 320 is the intersection line betweenthe two 3D surfaces and corresponds to line 310 in FIG. 4D indicatingall combinations of beamforming angles θ, ψ providing equal SNRs to boththe first and the third communication links L1, L3. From thesecombinations, that one providing the highest SNR, which is defined bythe saddle or anticline between the two peaks of the two 3D surfaces, isselected as the multicast precoding matrix to maximize the SNR at equalQoS for both communication links. In other words, in case of two or morecommunication links L1, L2, L3 a beamforming setting is selected wherethe SNR of the at least two most inferior communication links are equaland maximal.

Instead of the lines 310, 320 indicating equal SNRs in FIGS. 4C and 4D,twisted ribbons including and extending along the lines 310, 320 mayindicate combinations of the beamforming angles θ, ψ with the SNRs ofthe concerned communication links deviating from each other by at most apredefined threshold. Accordingly, from these combinations that oneproviding the highest SNR is selected as the multicast precoding matrixto maximize the SNR at approximately equal QoS for both communicationlinks.

The embodiments described above refer to multicast transmission on twocommunication links. In the case of multicasting on three, the controlunit 110 of FIG. 4A may in one embodiment check, for each beam map,whether the peak indicating the maximum SNR is the highest SNR among allbeam maps at the same combination of beamforming angles θ, ψ. In thiscase typically at least one intersection point exists between all threebeam maps and the control unit 110 selects the beamforming angles θ, ψassigned to the intersection point or, in the case of more than oneintersection points, that one of the intersection points providing thehighest SNR.

In case of providing multicast transmission through more than threecommunication links the control unit 110 may identify at least two orall intersection points between the beam maps of three of thecommunication links. For each combination of beamforming angles θ, ψidentifying an intersection point, the control unit 110 identifies thelowest SNR of all beam maps. Among all identified lowest SNRs assignedto any of the intersection points, the control unit 110 identifies thehighest one and selects the corresponding combination of beamformingangles θ, ψ as entries of the multicast precoding matrix. In case ofmultiple beam maps where the highest SNR of the most inferiorcommunication link is not the highest SNR of all beam maps for therespective combination of beamforming angles θ, ψ, a combination of therules above may be applied.

According to another embodiment the control unit 110 applies aniterative approach for determining points in the beam maps for eachcommunication link selected for multicast transmission. For each beammap the algorithm may identify the combination of beamforming angles θ,ψ assigned to the maximum SNR of the respective beam map, i.e. thepeaks. Among the identified peaks the algorithm may identify that onewith the lowest SNR, i.e. the lowest peak. If all other beam mapsprovide a higher SNR for all combinations of beamforming angles, thenthe combination of beamforming angles θ, ψ assigned to the lowest peakare selected for providing the precoding matrix for multicasttransmission. Otherwise the combination of beamforming angles θ, ψ ofthe lowest peak provides start values of an iterative process.

From the other identified peaks the algorithm may identify that one withthe 2^(nd) lowest SNR, i.e. the 2^(nd) lowest peak. The combination ofbeamforming angles θ, ψ of the 2^(nd) lowest peak provides target valuesfor beamforming angles θ, ψ checked in course of the iterative process.

The iterative process checks the SNRs of beamforming angles θ, ψ betweenthe lowest peak and the 2^(nd) lowest peak. Starting from the startvalues identifying the lowest peak the iterative process modifies thebeamforming angles θ, ψ in the direction of the target valuesidentifying the 2^(nd) lowest peak by applying a preset step size andchecks for each modified combination of beamforming angles θ, ψ thecorresponding SNRs of the beam maps with the lowest peak and the 2^(nd)lowest peak. If the SNRs deviate from each other by not more than apredetermined threshold, the current beamforming angles may be selectedfor determining the multicast precoding matrix. If the SNRs deviate fromeach other by more than a predetermined threshold, the modification isrepeated. If the difference between the SNRs has increased between twoiteration steps, the modification may be cancelled and the algorithm maybe repeated at a lower step size. According to another embodiment, theratio of the modification of both beamforming angles may be modified tocheck whether the assumed saddle between the lowest and the 2^(nd)lowest peak is offset to the direct connection line between the twopeaks. Otherwise the modification may be repeated until the SNRs of bothbeam maps are identical or differ from each other by not more than apreset value. The final value of the modification is selected fordetermining the entries of the precoding matrix.

Another embodiment of the communication device 100 gets by with minimumbeamforming information including not more than the beamforming anglesfor the optimum unicast communication link, which may be quantized, andat least coarse information about the SNR of the communication link atthe optimum beamforming angles. Typically this information is availablein standardized systems providing beamforming, for exampleEigenbeamforming or spot beamforming for unicast transmission. In suchsystems the receiving communication devices may feed back a tone map andoptimum beamforming angles or pointers into a LUT (look-up table)identifying the optimum beamforming angles, wherein different optimumbeamforming angles may be determined for different subcarrierfrequencies. The tone map may include, for at least two differentfrequency bands, an index identifying a preset range within which theSNR of the concerned communication link is when the optimum beamformingangles are applied for the respective frequency band.

Using the beamforming angles identifying the position of the peaks ofthe communication links selected for multicast transmission as startvalues, the control unit 110 may select a combination of beamformingangles θ, ψ between the peaks. By trial and error, the optimumcombination of the beamforming angles θ, ψ might be found by comparingthe constellation of the tone maps of the concerned communication links.The control unit 110 may select a combination of beamforming angles θ, ψshowing identical and maximal constellation or identical constellationand SNRs. Since the approach exploits not more than feedback informationdefined in existing standards, the approach can be implemented on top ofany standard, which already supports beamforming.

According to a further embodiment, the transmitting communication device100 tries various combinations of the beamforming angles θ, ψ and thereceiver may feedback information about the optimum beamforming anglesθ, ψ on the basis of the tried beamforming angles θ, ψ and the tone map.Using this information, the transmitting communication device 100derives another point of the beam map for a further trial. For example,the control unit 110 applies an iterative trial-and-error algorithm andtransmits test signals through the at least first and second channelsusing different precoding matrices. In response to the test signals eachof the concerned receiving communication devices sends updatedbeamforming information, e.g. the beamforming angles for a precodingmatrix optimized for the new equivalent channel and the SNR for theupdated beamforming angles. The control unit 110 evaluates the updatedbeamforming information. The control unit 110 tries new precodingmatrixes until the updated SNRs deviate from each other by less than apreset value.

A further embodiment refers to a preset LUT linking the best combinationof the beamforming angles θ, ψ for multicast transmission with actualbeamforming angles θ, ψ and SNR information or tone maps of eachcommunication link. For each total number of multicast communicationlinks another LUT might be provided. For example another LUT may be usedfor a multicast communication with two receiving communication devicesthan for a multicast communication with three receiving communicationdevices. The pointers accessing the concerned LUT may be informationfrom the tone maps and the optimum beamforming angles of the concernedcommunication links. The entries and the output of the LUTs give themulticast beamforming angles θ, ψ. According to an embodiment, the LUTsmight be designed offline, wherein the LUTs might be based on a set ofmeasured exemplary transmission channels. According to anotherembodiment, the LUTs are gathered in a training mode of the transmittingcommunication device 100 in the operational environment. As thisapproach does not require any additional feedback from the receivers,the approach can be implemented without changing existing standardssupporting beamforming.

FIG. 5 refers to an embodiment with the transmitting communicationdevice 100 providing beamforming for several frequency carriers. A datasource, for example a processor unit 120, outputs a primary data streamthat contains payload data. An FEC (forward error correction) unit 130may insert code redundancy according to an error detection scheme forfacilitating error correction at the receiver side and outputs a firstdata stream d1. In case the transmitting communication device 100provides Eigenbeamforming, a stream parser unit 135 may split up thefirst data stream d1 into at least two complementary data streams d2 ormay multiply, at least double, the first data stream d1 into two or moreidentical data streams d2 and provides the second data streams d2 to amodulator unit 140. In case of spot beamforming or in a SISO(single-input-single-output) mode the parser unit 135 may directlyforward the first data stream d1 as the second data stream d2 to themodulator unit 140.

A modulator unit 140 modulates the data stream d2, for example, by usinga plurality of sub-carriers and QAMs (quadrature amplitude modulation),respectively. The modulator unit 140 may use constellation data ConDATresulting from frequency dependent channel characteristics and adaptingthe QAM scheme to the respective transmission channel. A control unit110 may derive the constellation data ConDAT from feedback informationwhich may be received, for example, through a receiver unit receivingsignals via the same transmission channel through which the transmittingcommunication device 100 transmits signals.

A precoder unit 150 receives signals corresponding to the one or moremodulated data streams and precodes the signals using a precoding matrixV provided by the control unit 110. In the case of multicasttransmission the control unit 110 obtains multicast precodinginformation descriptive for a multicast precoding matrix from evaluatingbeamforming information from all transmission channels for which themulticast transmission is applied, wherein the beamforming informationmay be at least partly contained in the received tone map information.The control unit 110 may be one as described above and may provide amulticast precoding matrix allowing, for in substance balancedcommunication links, approximately equal QoS for all communication linksselected for multicast transmission.

An OFDM modulator unit 160 may modulate the precoded (beamformed)signals on a frequency carrier using OFDM (orthogonal frequency divisionmodulation) and inverse fast Fourier transformation to combine theorthogonal signals for obtaining digital signals describing thetransmission signals in the time domain. A transmitter circuit 190 maycomprise a mixer to shift the base-band signal to a useable frequencyrange and a DAC (digital to analogue converter) to convert the digitalsignals into analog transmission signals. The transmitter circuit 190couples each analog transmission signal to a corresponding transmit port101, 102. The transmit ports 101, 102 may be antennas or wire connectionblocks, by way of example.

The flowchart of FIG. 6 illustrates a method of operating acommunication device. The method includes providing multicast precodinginformation from at least first beamforming information descriptive fora first transmission channel and second beamforming informationdescriptive for a second transmission channel (602). At least one signalis beamformed using a multicast precoding information, wherein at leasttwo precoded signals are obtained (604).

Through a transmitter circuit that is electrically coupled to theprecoder unit transmission signals derived from the precoded signals aremulticast through the at least first and second transmission channels(606).

Obviously, numerous modifications and variations of the presentdisclosure are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

The invention claimed is:
 1. A communication device comprising: acontrol circuit configured to obtain multicast precoding informationfrom at least first beamforming information for a first transmissionchannel and second beamforming information for a second transmissionchannel, the first beamforming information including a first set ofbeamforming angles corresponding to a first maximum SNR (signal-to-noiseratio) for the first transmission channel and the second beamforminginformation including a second set of beamforming angles correspondingto a second maximum SNR for the second transmission channel; a precodercircuit configured to beamform at least one signal using the multicastprecoding information to obtain at least two precoded signals; and atransmitter circuit configured to multicast transmission signals derivedfrom the at least two precoded signals through the at least first andsecond transmission channels, wherein the transmitter circuit iselectrically coupled to the precoder circuit.
 2. The communicationdevice according to claim 1, wherein each of the first and secondbeamforming information relates one or more candidate precoding matricesto a channel quality criterion resulting by applying the respectivecandidate precoding matrix to the respective transmission channel. 3.The communication device according to claim 2, wherein the controlcircuit is configured to: identify candidate matrices providing channelquality criteria that are equal for the at least first and secondtransmission channels when the first transmission channel has a highervalue of a criterion of the channel quality criteria than the secondtransmission channel for at least a first matrix of the candidatematrices, and the second transmission channel has a higher value of acriterion of the channel quality criteria than the first transmissionchannel for at least a second matrix of the candidate matrices, andselect the candidate matrices as the multicast precoding information,and the channel quality criteria for the first and second transmissionchannels includes at least one of SNR, a number indicating a range ofthe SNR, a maximum data throughput rate, BER (bit error rate) and anumber indicating a range of the BER.
 4. The communication deviceaccording to claim 3, wherein the channel quality criteria of the atleast first and second transmission channels are equal whencorresponding SNRs (signal-to-noise ratios) for the at least first andsecond transmission channels deviate from each other by not more than agiven threshold.
 5. The communication device according to claim 4,wherein the given threshold is 0.5 dB.
 6. The communication deviceaccording to claim 3, wherein the control circuit is configured toobtain the multicast precoding information by selecting, from thecandidate matrices providing equal channel quality criterions, acandidate matrix providing the best channel quality criterion.
 7. Thecommunication device according to claim 1, wherein the precoder circuitis configured to perform spot beamforming of the signal.
 8. Thecommunication device according to claim 1, comprising the controlcircuit is configured to: determine whether a result of subtractingvalues of SNRs for the first transmission channel from values of SNRSfor the second transmission channel is positive or negative, identify acandidate matrix providing equal and maximal SNRs for at least inferiortransmission channel of the at least first and second transmissionchannels, and select the candidate matrix as the multicast precodinginformation, wherein the first transmission channel is inferior when theresult is negative and the second transmission channel is inferior whenthe result is positive.
 9. The communication device according to claim1, wherein each of the at least first and second beamforming informationcontains a information for a specific precoder matrix for a unicasttransmission through the respective transmission channel and acorresponding SNR information.
 10. The communication device according toclaim 9, wherein the control circuit is configured to perform aniterative algorithm searching between SNR maxima of two communicationlinks for a precoding matrix providing equal SNRs for the twocommunication links.
 11. The communication device according to claim 10,wherein the two communication links are the communication links with thelowest and the second lowest maximum SNR among all communication linksselected for multicast transmission.
 12. The communication deviceaccording to claim 1, wherein each of the at least first and secondbeamforming information contains information for at least one furtherprecoder matrix and a corresponding SNR resulting from a unicasttransmission through the respective transmission channel using the atleast one precoder matrix.
 13. The communication device according toclaim 1, wherein each of the at least first and second beamforminginformation contains information for at least all other evaluatedunicast precoder matrices and corresponding SNR information.
 14. Thecommunication device according to claim 1, wherein at least one of theat least first and second beamforming information is contained in anOFDM (orthogonal frequency division multiplexing) feedback signalreceived by a receiver circuit, and the OFDM feedback signal containstone map that includes an index identifying a preset range of the SNRsfor at least two different frequency bands of the at least first andsecond transmission channels.
 15. The communication device according toclaim 1, wherein the communication device is a PLC (power linecommunication) device and the transmitter circuit is configured tomulticast the transmission signals through power lines.
 16. Thecommunication device according to claim 1, wherein the control circuitis configured to apply an iterative trial-and-error method providingtransmitting signals through the at least first and second channelsusing different precoding matrices and evaluating further the at leastfirst and second beamforming information received in response to thesignals.
 17. A communication device comprising: a receiver circuit forreceiving at least multicast transmission signals; a decoder circuitconfigured to decode the at least transmission signals to obtain atleast one decoded data signal; and a transmitter circuit configured totransmit expanded beamforming information in response to a signalreceived through the receiver circuit, the expanded beamforminginformation including at least set of beamforming angles correspondingto a maximum SNR, and location and value of the maximum SNR andidentifying location and SNR value of at least two points of a 3Dsurface in a beam map, and the beam map indicating SNRs for eachcombination of the set of beamforming angles in each of communicationlinks between the communication device and other communication devices.18. A communication system comprising at least three communicationdevices, wherein at least one of the at least three communicationdevices comprises a control circuit configured to obtain multicastprecoding information from at least first beamforming information for afirst transmission channel and second beamforming information for asecond transmission channel, the first beamforming information includinga first set of beamforming angles corresponding to a first maximum SNR(signal-to-noise ratio) for the first transmission channel and thesecond beamforming information including a second set of beamformingangles corresponding to a second maximum SNR for the second transmissionchannel; a precoder circuit configured to beamform at least one signalusing the multicast precoding information to obtain at least twoprecoded signals; and a transmitter circuit configured to multicasttransmission signals derived from the at least two precoded signalsthrough the at least first and second transmission channels, wherein thetransmitter circuit is electrically coupled to the precoder circuit. 19.A method of operating a communication device, the method comprising:obtaining, by a control circuit of the communication device, multicastprecoding information from at least first beamforming information for afirst transmission channel and second beamforming information for asecond transmission channel, the first beamforming information includinga first set of beamforming angles corresponding to a first maximum SNR(signal-to-noise ratio) for the first transmission channel and thesecond beamforming information including a second set of beamformingangles corresponding to a second maximum SNR for the second transmissionchannel; beamforming, by a precoder circuit of the communication device,at least one signal using the multicast precoding information to obtainat least two precoded signals; and multicasting, by a transmittercircuit of the communication device transmission signals through the atleast first and second transmission channels, the multicast transmissionsignals derived from the at least two precoded data signals, thetransmitter circuit electrically coupled to the precoder circuit. 20.The method according to claim 19, wherein each of the at least first andsecond beamforming information relates one or more candidate precodingmatrices to a channel quality criterion resulting by applying therespective candidate precoding matrix to the respective transmissionchannel.
 21. A communication device comprising circuitry configured to:obtain multicast precoding information from at least first beamforminginformation for a first transmission channel and second beamforminginformation for a second transmission channel, the first beamforminginformation including a first set of beamforming angles corresponding toa first maximum SNR (signal-to-noise ratio) for the first transmissionchannel and the second beamforming information including a second set ofbeamforming angles corresponding to a second maximum SNR for the secondtransmission channel; beamform at least one signal using the multicastprecoding information to obtain at least two precoded signals; andmulticast transmission signals derived from the at least two precodedsignals through the at least first and second transmission channels.